1. Field of the Invention
The present invention relates to a liquid crystal display apparatus using a liquid crystal modulation element, such as a liquid crystal projector.
2. Description of the Related Art
Some of the liquid crystal modulation elements are realized by sealing nematic liquid crystal having positive dielectric anisotropy between a first transparent substrate having a transparent electrode (common electrode) formed thereon and a second transparent substrate having a transparent electrode (pixel electrode) forming pixels, wiring, switching elements and the like formed thereon. The liquid crystal modulation element is referred to as a Twisted Nematic (TN) liquid crystal modulation element in which the major axes of liquid crystal molecules are twisted by 90 degrees continuously between the two glass substrates. This liquid crystal modulation element is used as a transmissive liquid crystal modulation element. Some of the liquid crystal modulation elements utilize a circuit substrate having reflecting mirrors, wiring, switching elements and the like formed thereon instead of the abovementioned second transparent substrate. This is called a Vertical Alignment (Arrangement) Nematic (VAN) liquid crystal modulation element in which the major axes of liquid crystal molecules are aligned in homeotropic alignment substantially perpendicularly to two substrates. The liquid crystal modulation element is used as a reflective liquid crystal modulation element.
In these liquid crystal modulation elements, typically, the Electrically Controlled Birefringence (ECB) effect is used to provide retardation for a light wave passing through a liquid crystal layer to control the change of polarization of the light wave, thereby forming an image with light.
The liquid crystal modulation element utilizes the ECB effect to modulate the light intensity, and the application of an electric field to the liquid crystal layer moves charged particles (ionic substances) present in the liquid crystal layer. When a DC electric field is continuously applied to the liquid crystal layer, the charged particles are drawn toward one of two opposite electrodes. Even when a constant voltage is applied to the electrodes, the electric field substantially applied to the liquid crystal layer is attenuated or increased by the charge of the charged particles.
To avoid such a phenomenon, a line inversion drive method is typically employed which inverts the polarity of an applied electric field between positive and negative polarities for each line of arranged pixels and changes the polarity in a predetermined cycle such as 60 Hz or the like. In addition, a field inversion drive method is also used which inverts the polarity of an electric field applied to all arranged pixels between positive and negative in a predetermined cycle. These drive methods can avoid the application of the electric field of only one polarity to the liquid crystal layer to prevent unbalanced ions.
However, the liquid crystal layer, and an outer wall member surrounding the liquid crystal layer and the like also include there inside charged particles. When the liquid crystal is driven in a high temperature environment in particular, these charged particles drift (or move) in the liquid crystal layer. These charged particles generate a DC electric field component in the liquid crystal layer, and attach to an interface between the liquid crystal layer and an alignment film or an electrode. Then, the charged particles drift and accumulate in a direction along which the liquid crystal molecules are aligned.
In a liquid crystal modulation element having an organic alignment film, in addition to the drifting of charged particles due to the driving of the liquid crystal under the high temperature environment, light entering the liquid crystal modulation element causes decomposition of organic materials forming the alignment film, the liquid crystal, a seal member or the like, causing the generation of charged particles. These charged particles also generate a DC electric field component in the liquid crystal layer, attach to the interface between the liquid crystal layer and the alignment film or the electrode, and then drift and accumulate in the direction along which the liquid crystal molecules are aligned.
The charged particles that have accumulated in a specific area in the liquid crystal layer change the effective electric field applied to the liquid crystal layer, thereby preventing an expected ECB modulation. This causes, for example, luminance unevenness in an effective display area of the liquid crystal modulation element, which deteriorates image quality.
Countermeasures applied against such a problem have been disclosed in International Publication No. 2005-012986 pamphlet and Japanese Patent Laid-Open Nos. 8-201830, 11-38389, and 5-323336.
International Publication No. 2005-012986 pamphlet discloses a method in which at least one of potentials of the pixel electrode and the electrode opposite thereto of a liquid crystal cell is set to a ground level during a period other than an image display operation period such that ions causing a burn-in phenomenon are dissociated from the interface between the liquid crystal layer and the alignment film or the electrodes.
Japanese Patent Laid-Open No. 8-201830 discloses a method in which an ion trap electrode area is provided in a non-display area of a liquid crystal modulation element, and a direct voltage is applied to the ion trap electrode such that ionic impurities are absorbed by the ion trap electrode area of the non-display area having no influence on image display.
Japanese Patent Laid-Open No. 11-38389 discloses a method in which a metal film electrode is provided at a position different from that of the pixel electrode to apply a direct voltage between the metal film electrode and a common electrode, thereby reducing the concentration of movable ions in a display area to suppress a flicker phenomenon.
Furthermore, Japanese Patent Laid-Open No. 5-323336 discloses a method in which ion trap electrodes are provided independently of a transparent electrode at opposing surfaces of two electrode substrates provided in the vicinity of a liquid crystal enclosing portion, and a voltage is applied to the ion trap electrodes to trap ionic impurities.
As described above, the voltage control from the outside can control the charged particles in the liquid crystal modulation element to provide displayed images of good quality.
However, the method disclosed in International Publication No. 2005-012986 needs in a circuit of the liquid crystal modulation element a switching part for setting the potential of the opposite electrodes to the ground level. This increases the number of steps of manufacturing the liquid crystal modulation element. Furthermore, the setting of the potential of the opposite electrodes to the ground level is not sufficiently effective because forces for pulling off the ions that have attached to the interface of the liquid crystal layer and the alignment film or the electrode are weaker than coulomb forces.
Similarly, the methods disclosed in Japanese Patent Laid-Open Nos. 8-201830, 11-38389, and 5-323336 also need to newly provide the ion trap electrode for attracting the ions in the non-display area, so that the number of the manufacturing steps increases. Moreover, although in these disclosed methods the ionic impurities are drawn by the coulomb force, the coulomb force is inversely proportional to the square of a distance from the ion trap electrode, so that the ions generated at a position away from the ion trap electrode cannot be efficiently attracted.